1. Field of the Invention
The invention relates to compensation of a frequency drift of an oscillator.
2. Description of Related Art
European Patent Application EP-A 740,421 describes a temperature-compensated piezoelectric oscillator. It is adapted to compensate for temperature dependent frequency changes of a piezoelectric vibrator used as the oscillation frequency determining element. The oscillator comprises, in addition to the vibrator, an oscillator circuit for driving the vibrator, a temperature sensor circuit for sensing the ambient temperature, and a variable power supply circuit for applying, to the oscillator circuit, a power supply voltage VDD(T) which is controlled in response to the sensed temperature by the temperature sensor circuit in such a way as to compensate the frequency-temperature characteristic of the vibrator.
If the power supply voltage supplied to an oscillator is varied, this will not only influence the frequency but also the performance of the oscillator in terms of, for example, phase noise. Consequently, if a frequency drift of the oscillator is compensated by varying the power supply voltage, there is a risk that the performance may become unacceptably low. The risk will be serious, particularly, if the supply voltage needs to be varied to a relatively large extent in order to achieve a satisfactory frequency-drift compensation. It may even be the case that, given a maximum level of the power supply voltage, the extent to which the power supply voltage can be varied is too small to achieve a satisfactory frequency-drift compensation without deteriorating the oscillator's performance.